1. Field of the Invention
The present invention relates to a liquid crystal display device wherein liquid crystal molecules are aligned substantially in parallel with a substrate surface by rotation control.
2. Description of the Related Art
Liquid crystal display devices using an in-plane switching type are well known. A liquid crystal layer of these devices is provided between a pair of substrates placed opposite to each other at a given distance, ensuring that liquid crystal molecules are aligned substantially in parallel to a substrate surface, with their long axes aligned in a certain direction. On the internal surface of one of the substrate pair, pixel electrodes and a common electrode are formed, insulated from each other, to generate electric field. The electric field controls each liquid crystal molecule to align substantially in parallel to the substrate surface. For example, a thin-film transistor is placed in each region enclosed with top and bottom scanning lines and right and left data lines, a control electrode of the thin-film transistor is connected to a scanning line, an input electrode of the thin-film transistor is connected to a data line, and an output electrode of the thin-film transistor is connected to the pixel electrode. Data lines and pixel electrodes are formed on an interlevel insulating layer containing a gate insulating film. The common electrode is provided above pixel electrodes and the interlevel insulating layer via another interlevel insulating layer. The common electrode has a plurality of slits, namely openings penetrating in the thickness direction. The method of generating an in-plane electric field between the edge of each slit and the solid pixel electrode may be called the fringe field switching (FFS) method. In FFS method, a structure extending slits in parallel to a scanning line or data line and a dog-leg structure extending not in parallel to a scanning line or data line (FIG. 12 of JP2007-233317 and JP2005-107535) are known. With the latter, the electric field applied between the common electrode and the pixel electrode acts in the direction obliquely crossing the long axes of the liquid crystal molecules in the initial alignment state, aligning the direction of the liquid crystal in the initial alignment state and thus ensuring uniform brightness. In addition, this structure forms multiple domains that divide the alignment direction of liquid crystal molecules in one pixel, further expanding viewing angle, which is why this structure is often adopted.
In the liquid crystal display device having a non-parallel structure described above, the area enclosed with the top and bottom scanning lines and the left and right data lines is approximately in a rectangular shape, whereas the slits have the shape of a hanger or a dog leg, not a long rectangular shape extending in the column direction. Consequently, if slits are arranged on the common electrode in the column or row direction, there is some area on which hanger or dog-leg-shaped openings cannot be formed. Liquid crystal molecules existing in this area, namely a residual area, are relatively not affected by an in-plane electric field generated between the pixel electrode and the common electrode, and therefore do not contribute to display. As a result, not only transmittance from the back side to the front side of the liquid crystal display device but also aperture ratio itself decreases, which is a problem. The same problem arises when slits are formed linearly but not in parallel with the column or row direction.